Re: [PATCH-for-9.1 v2 2/3] migration: Remove RDMA protocol handling

[Dr. David Alan Gilbert]

* Michael Galaxy (mgalaxy@akamai.com) wrote:
> One thing to keep in mind here (despite me not having any hardware to test)
> was that one of the original goals here
> in the RDMA implementation was not simply raw throughput nor raw latency,
> but a lack of CPU utilization in kernel
> space due to the offload. While it is entirely possible that newer hardware
> w/ TCP might compete, the significant
> reductions in CPU usage in the TCP/IP stack were a big win at the time.
> 
> Just something to consider while you're doing the testing........

I just noticed this thread; some random notes from a somewhat
fragmented memory of this:

  a) Long long ago, I also tried rsocket; 
      https://lists.gnu.org/archive/html/qemu-devel/2015-01/msg02040.html
     as I remember the library was quite flaky at the time.

  b) A lot of the complexity in the rdma migration code comes from
    emulating a stream to carry the migration control data and interleaving
    that with the actual RAM copy.   I believe the original design used
    a separate TCP socket for the control data, and just used the RDMA
    for the data - that should be a lot simpler (but alas was rejected
    in review early on)

  c) I can't rememmber the last benchmarks I did; but I think I did
    manage to beat RDMA with multifd; but yes, multifd does eat host CPU
    where as RDMA barely uses a whisper.

  d) The 'zero-copy-send' option in migrate may well get some of that
     CPU time back; but if I remember we were still bottle necked on
     the receive side. (I can't remember if zero-copy-send worked with
     multifd?)

  e) Someone made a good suggestion (sorry can't remember who) - that the
     RDMA migration structure was the wrong way around - it should be the
     destination which initiates an RDMA read, rather than the source
     doing a write; then things might become a LOT simpler; you just need
     to send page ranges to the destination and it can pull it.
     That might work nicely for postcopy.

Dave

> - Michael
> 
> On 5/9/24 03:58, Zheng Chuan wrote:
> > Hi, Peter，Lei，Jinpu.
> > 
> > On 2024/5/8 0:28, Peter Xu wrote:
> > > On Tue, May 07, 2024 at 01:50:43AM +0000, Gonglei (Arei) wrote:
> > > > Hello,
> > > > 
> > > > > -----Original Message-----
> > > > > From: Peter Xu [mailto:peterx@redhat.com]
> > > > > Sent: Monday, May 6, 2024 11:18 PM
> > > > > To: Gonglei (Arei) <arei.gonglei@huawei.com>
> > > > > Cc: Daniel P. Berrangé <berrange@redhat.com>; Markus Armbruster
> > > > > <armbru@redhat.com>; Michael Galaxy <mgalaxy@akamai.com>; Yu Zhang
> > > > > <yu.zhang@ionos.com>; Zhijian Li (Fujitsu) <lizhijian@fujitsu.com>; 
> > > > > Jinpu Wang
> > > > > <jinpu.wang@ionos.com>; Elmar Gerdes <elmar.gerdes@ionos.com>;
> > > > > qemu-devel@nongnu.org; Yuval Shaia <yuval.shaia.ml@gmail.com>; Kevin 
> > > > > Wolf
> > > > > <kwolf@redhat.com>; Prasanna Kumar Kalever
> > > > > <prasanna.kalever@redhat.com>; Cornelia Huck <cohuck@redhat.com>;
> > > > > Michael Roth <michael.roth@amd.com>; Prasanna Kumar Kalever
> > > > > <prasanna4324@gmail.com>; integration@gluster.org; Paolo Bonzini
> > > > > <pbonzini@redhat.com>; qemu-block@nongnu.org; devel@lists.libvirt.org;
> > > > > Hanna Reitz <hreitz@redhat.com>; Michael S. Tsirkin <mst@redhat.com>;
> > > > > Thomas Huth <thuth@redhat.com>; Eric Blake <eblake@redhat.com>; Song
> > > > > Gao <gaosong@loongson.cn>; Marc-André Lureau
> > > > > <marcandre.lureau@redhat.com>; Alex Bennée <alex.bennee@linaro.org>;
> > > > > Wainer dos Santos Moschetta <wainersm@redhat.com>; Beraldo Leal
> > > > > <bleal@redhat.com>; Pannengyuan <pannengyuan@huawei.com>;
> > > > > Xiexiangyou <xiexiangyou@huawei.com>
> > > > > Subject: Re: [PATCH-for-9.1 v2 2/3] migration: Remove RDMA protocol 
> > > > > handling
> > > > > 
> > > > > On Mon, May 06, 2024 at 02:06:28AM +0000, Gonglei (Arei) wrote:
> > > > > > Hi, Peter
> > > > > Hey, Lei,
> > > > > 
> > > > > Happy to see you around again after years.
> > > > > 
> > > > Haha, me too.
> > > > 
> > > > > > RDMA features high bandwidth, low latency (in non-blocking lossless
> > > > > > network), and direct remote memory access by bypassing the CPU (As 
> > > > > > you
> > > > > > know, CPU resources are expensive for cloud vendors, which is one of
> > > > > > the reasons why we introduced offload cards.), which TCP does not 
> > > > > > have.
> > > > > It's another cost to use offload cards, v.s. preparing more cpu 
> > > > > resources?
> > > > > 
> > > > Software and hardware offload converged architecture is the way to go 
> > > > for all cloud vendors
> > > > (Including comprehensive benefits in terms of performance, cost, 
> > > > security, and innovation speed),
> > > > it's not just a matter of adding the resource of a DPU card.
> > > > 
> > > > > > In some scenarios where fast live migration is needed (extremely 
> > > > > > short
> > > > > > interruption duration and migration duration) is very useful. To 
> > > > > > this
> > > > > > end, we have also developed RDMA support for multifd.
> > > > > Will any of you upstream that work?  I'm curious how intrusive would 
> > > > > it be
> > > > > when adding it to multifd, if it can keep only 5 exported functions 
> > > > > like what
> > > > > rdma.h does right now it'll be pretty nice.  We also want to make 
> > > > > sure it works
> > > > > with arbitrary sized loads and buffers, e.g. vfio is considering to 
> > > > > add IO loads to
> > > > > multifd channels too.
> > > > > 
> > > > In fact, we sent the patchset to the community in 2021. Pls see:
> > > > https://urldefense.com/v3/__https://lore.kernel.org/all/20210203185906.GT2950@work-vm/T/__;!!GjvTz_vk!VfP_SV-8uRya7rBdopv8OUJkmnSi44Ktpqq1E7sr_Xcwt6zvveW51qboWOBSTChdUG1hJwfAl7HZl4NUEGc$
> > Yes, I have sent the patchset of multifd support for rdma migration by 
> > taking over my colleague, and also
> > sorry for not keeping on this work at that time due to some reasons.
> > And also I am strongly agree with Lei that the RDMA protocol has some 
> > special advantages against with TCP
> > in some scenario, and we are indeed to use it in our product.
> > 
> > > I wasn't aware of that for sure in the past..
> > > 
> > > Multifd has changed quite a bit in the last 9.0 release, that may not 
> > > apply
> > > anymore.  One thing to mention is please look at Dan's comment on possible
> > > use of rsocket.h:
> > > 
> > > https://urldefense.com/v3/__https://lore.kernel.org/all/ZjJm6rcqS5EhoKgK@redhat.com/__;!!GjvTz_vk!VfP_SV-8uRya7rBdopv8OUJkmnSi44Ktpqq1E7sr_Xcwt6zvveW51qboWOBSTChdUG1hJwfAl7HZ0CFSE-o$
> > > 
> > > And Jinpu did help provide an initial test result over the library:
> > > 
> > > https://urldefense.com/v3/__https://lore.kernel.org/qemu-devel/CAMGffEk8wiKNQmoUYxcaTHGtiEm2dwoCF_W7T0vMcD-i30tUkA@mail.gmail.com/__;!!GjvTz_vk!VfP_SV-8uRya7rBdopv8OUJkmnSi44Ktpqq1E7sr_Xcwt6zvveW51qboWOBSTChdUG1hJwfAl7HZxPNcdb4$
> > > 
> > > It looks like we have a chance to apply that in QEMU.
> > > 
> > > > 
> > > > > One thing to note that the question here is not about a pure 
> > > > > performance
> > > > > comparison between rdma and nics only.  It's about help us make a 
> > > > > decision
> > > > > on whether to drop rdma, iow, even if rdma performs well, the 
> > > > > community still
> > > > > has the right to drop it if nobody can actively work and maintain it.
> > > > > It's just that if nics can perform as good it's more a reason to 
> > > > > drop, unless
> > > > > companies can help to provide good support and work together.
> > > > > 
> > > > We are happy to provide the necessary review and maintenance work for 
> > > > RDMA
> > > > if the community needs it.
> > > > 
> > > > CC'ing Chuan Zheng.
> > > I'm not sure whether you and Jinpu's team would like to work together and
> > > provide a final solution for rdma over multifd.  It could be much simpler
> > > than the original 2021 proposal if the rsocket API will work out.
> > > 
> > > Thanks,
> > > 
> > That's a good news to see the socket abstraction for RDMA!
> > When I was developed the series above, the most pain is the RDMA migration 
> > has no QIOChannel abstraction and i need to take a 'fake channel'
> > for it which is awkward in code implementation.
> > So, as far as I know, we can do this by
> > i. the first thing is that we need to evaluate the rsocket is good enough 
> > to satisfy our QIOChannel fundamental abstraction
> > ii. if it works right, then we will continue to see if it can give us 
> > opportunity to hide the detail of rdma protocol
> >      into rsocket by remove most of code in rdma.c and also some hack in 
> > migration main process.
> > iii. implement the advanced features like multi-fd and multi-uri for rdma 
> > migration.
> > 
> > Since I am not familiar with rsocket, I need some times to look at it and 
> > do some quick verify with rdma migration based on rsocket.
> > But, yes, I am willing to involved in this refactor work and to see if we 
> > can make this migration feature more better:）
> > 
> > 
> 
-- 
 -----Open up your eyes, open up your mind, open up your code -------   
/ Dr. David Alan Gilbert    |       Running GNU/Linux       | Happy  \ 
\        dave @ treblig.org |                               | In Hex /
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